Why can't wing-mounted spoilers be used to steepen approaches?

Question

In a comment on this answer to my previous question about tail-mounted airbrakes (lack of), @JohnK states:

You won't see flight spoilers (speed brakes) being used to steepen approaches. You have to be 10kt above Vref to extend them on a CRJ900 and you shouldn't be that hot in the first place, and you aren't allowed to use them at all below 300 ft.

Why do you have to be going abnormally fast to slow down with the spoilers, and why can't they be used at low altitude (especially given that, just like with airbrakes, it would seem to me like spoiler extension would be a safer method of slowing down than pulling the throttle all the way back would be, since, should a go-around become necessary, the spoilers can retract a lot more quickly than the engines can spool up from idle to TOGA)?

Answer 1

They can. The Airbus A318 and Embraer E190 both do this in steep approach mode. You can see spoilers extended in this video of a A318 landing and a similar approach in an E190 at London City Airport.

Going back further, the Lockheed L-1011 did this with a system called Direct Lift Control. When landing flaps were selected, spoilers raised a set amount. Small movements on the yoke raised or lowered the spoilers instead of adjusting the elevators. This made for a very smooth approach and landing.

The common point is that these aircraft use electronics for spoiler control. It's too complicated to do this in a cable flight control system. You need to have the system retract the spoilers quickly in cases like go-around and stalls. As I recall, steep approach mode on the A318 activates a different set of gains on the flight control loops for better handling qualities in that configuration.

Finally, a number of fighters have the same system, and some glider pilots do this manually.

Answer 2

The main issue is the need to manage the vertical sink rate of your 100000+ lb aluminum trash can and the risks of hard landings.

Although they are often called speed brakes, the main effect of flight spoilers is to increase sink rate (without speeding up). On a landing approach at typical jet speeds of around 130kt you don't want to be descending any more than about 700-800 fpm (about what you get at that speed on a 3 degree slope). More than 1000 fpm is typically considered an unstablilized approach and necessitates a go around if at that sink rate below 1000 ft with many airlines.

In the landing configuration on a 3 degree ILS glide slope you are flying 99.9% of the time, you will normally need considerable thrust on, and small pitch and thrust adjustments are more than sufficient to precisely maintain the slope and sink rate.

The problem is having high sink rates close to the ground and having to arrest the sink at the bottom with all that mass/intertia doing downhill, when the only tool you have to do so is to increase overall lift by pitching up. Once the high sink is established, retracting the flight spoilers only helps a little and you have to pitch up (and add thrust, but that it is going to be delayed).

On airliners with leading edge slats and multi-slotted flaps that are super draggy in the landing configuration, unless you have a significant energy margin above Vref, the ability to arrest the sink rate with pitch is limited and there is little margin for error. Get a little bit below Vref and pull pitch, and not much happens until the thrust comes up. As a result there is a high risk that you'll continue sinking right through the landing flare and hit hard.

Answer 3

Spoilers are called spoilers, because they work by spoiling lift.

To slow the aircraft, or make it descend faster, it needs to dissipate the energy and that means increase drag. Disrupting the airflow over the wing creates a lot of drag due to stagnation behind the spoilers, and additional induced drag as the lift distribution is pushed further from optimal. After landing disrupting lift also puts more weight on the wheels, which makes the brakes more effective.

The price for it is, however, is that the aircraft flies at higher angle of attack to compensate for the lift reduction, and that means closer to stall. That means the aircraft must not fly as slow to maintain the safety margin, and shouldn't use spoilers at low altitude where there would not be enough height to recover in case of stall.

Gliders to normally use spoilers all the way to landing. Without engine, it is their only way to regulate the rate of losing energy. However a light slow glider can afford flying close to stall, because they can be recovered very quickly after retracting the spoilers. Their spoilers also tend to be relatively small, so they don't allow that steep approaches.

Airliners are another matter though. They have a lot of inertia, which makes them respond to control input rather slowly. Combined with their much higher speed it makes stall due to spoilers left out a bit too long a much bigger problem. Their spoilers are also larger as their primary functions are dumping lift on the ground and emergency descent, which is normally flown at maximum allowable speed.

As mentioned by @user71659, there are some aircraft that do use spoilers for steep approach, e.g. A318. But A318 has unusually low stall speed as it uses the same wings as A320 that are large for its weight, and its flight control computer is programmed to retract the spoilers (and spool up the engines) if it gets too close to stall.

It would be also possible to have the spoiler extend only a little bit, or only some of them, for steeper approaches. However, steep approaches are rare, so they are not worth the additional complexity in the control system for the purpose for most aircraft manufacturers and airlines.

Answer 4

The answer is yes, you CAN use spoilers (in a flight position!) to steepen an approach. The aircraft remains fully controllable and can be flared prior to touchdown with the spoilers in this position.

While it is safe, legal, and reasonable to do this, it’s considered a faux pas among professional airline and corporate pilots as high performance flying interferes with passenger comfort, especially for nervous flyers. Making the trip as uneventful and smooth as possible is part of a pilot’s job as well.

Answer 5

What it boils down to is brakes (aerodynamic, wheels, and reverse thrusters) and weight.

First the strategy for the massive airliner: come in as flat and slow as possible, set it down, and bring 200 tons to a stop. There for the best application of airbrakes is to have them as lifting devices. Slats and flats create an under and over cambered wing, which, while draggy (good here), produces the most lift at slow speeds. Steep approaches are out as the physics of inertia make this a "case study" of the dangers of rounding out at low altitude and crushing the landing gear with too high a rate of descent. Set it down easy, then brakes, reverse thrust, every airbrake available (including spoilers). A plane landing at 150 knots requires more than 2 times the braking force as a plane landing at 100 knots.

With a spoiler, you have to go faster to get the same amount of lift. With a flap, slower. Same drag. So in this case, you pivot the airbrake down.

Now for the sailplane strategy: steepen the angle of descent. If it is 50 feet high at the runway threshold it may sail right over a 3000 foot runway! We want drag and less lift, but how do we avoid stalling? By speeding up. Here, a little extra speed is no problem. Now you get the same lift at higher speed if you need it, rather than pitching up. Going faster in any case means less efficient gliding and more efficient air braking. The extra speed will not be a significant factor once the plane is down (getting it to stop).

So in this case, you pivot the airbrake up.

Same airbrake! Heavy, slow, flat - down. Light, fast, steep - up.

Depends on what you fly.